What is the aurora?

The solar wind -- a flow of charged particles boiling off and streaming away from the sun -- causes the effect. The same wind, at its most intense, disturbs satellite, radar and radio communications.

When the solar particles hit the earth's magnetic field, the magnetic forces deflect them toward the poles. The particles enter the upper atmosphere in paired oval rings centered on the north and south geomagnetic poles. There, they interact with the

dilute molecules of the upper air, creating sparks of a sort.

The process, on an atomic level, is similar to the way cathode ray tubes behind color television or computer screens generate images.

The different colors of the northern (and southern) lights come from the different atmospheric gases and their electrical states.

Oxygen about 60 miles up produces the most common yellow-green light. High-altitude oxygen (about 200 miles up) and neutral nitrogen glow red. Ionized nitrogen molecules make blue.

Views of auroras sweeping the horizon or the mountaintops are optical illusions. They are as far above as orbiting space craft, stretching from 40 to 600 miles above the earth.

Auroras are always present, but their intensity varies with the solar wind. The most visible northern lights are linked with 11-year sunspot cycles on the sun. The last peak was in 2000. Solar "storms" and flares also increase the particle stream. Gusts of solar wind plus the electrical and magnetic energy of the aurora twist the Earth's magnetic field lines, causing the curtains of light to brighten, fade, ripple and curl.

When the solar wind is at its strongest, the particles penetrate deeper into the magnetic field, making lights visible farther from the poles than usual. On rare occasions, people in other states in the U.S. can see the displays.

Whether auroras make sound remains an open question. Scientists say the aurora displays are too far away and in too thin air to make sound themselves, but some still-unknown electromagnetic effect on the ground below could generate noises.

People only began to understand the aurora in the 1930s. Today, the Geophysical Institute of the University of Alaska Fairbanks is in the forefront of international research on the northern lights. It posts a daily auroral forecast on the Internet at: www.gi.alaska.edu/cgi-bin/predict.cgi.